Purlins are the horizontal steel members that sit on top of the main rafters and hold up your roof sheets. They look like a small part of the building, but the wrong purlin size or spacing can cause sagging roofs, leaks and, in strong storms, roof failure.

This guide explains the difference between C and Z purlins, the common purlin sizes used in Bangladesh, and how to choose the right one for your factory or warehouse.

What Is a Purlin?

In a pre-engineered building (PEB), loads travel in a simple chain: roof sheet → purlin → rafter → column → foundation. Purlins carry the weight of the roof sheets, rain, maintenance workers and, most importantly in Bangladesh, the uplift force from cyclone and nor'wester winds. The same cold-formed sections used on walls are called girts.

C Purlin vs Z Purlin: What Is the Difference?

C Purlin (C Section)

Shaped like the letter C, with both flanges facing the same direction.
Easy to fix and handle, and good for simple single spans.
Commonly used for wall girts, small sheds, canopies, mezzanine framing and shorter bay spacing.

Z Purlin (Z Section)

Shaped like the letter Z, with flanges facing opposite directions.
Z sections can overlap (lap) at the rafters, making the purlin line act as a continuous beam. This makes them stronger and more economical over long, multi-bay buildings.
The preferred choice for the roofs of most factories and warehouses.

In short: use Z purlins for long, multi-bay roofs and C purlins for walls, short spans and secondary framing.

Common Purlin Sizes in Bangladesh

Purlins are described by depth × thickness. Typical ranges used in industrial buildings are:

Depth: 150 mm, 175 mm, 200 mm, 250 mm and 300 mm.
Thickness: about 1.5 mm to 3.0 mm.
Flange width: usually 60 mm to 80 mm, with a stiffening lip.

Deeper and thicker purlins are needed as bay spacing (the distance between frames) increases. For example, a building with 6 m bays can often use a lighter section than one with 8 m or 9 m bays. The exact size must come from structural design, checked for dead load, live load and wind load under BNBC 2020.

Purlin Spacing

Purlin spacing on industrial roofs is commonly in the range of 1.2 m to 1.5 m, depending on the roof sheet thickness and profile. Thinner sheets need closer purlins. Closer spacing also helps at the eaves, ridge and roof edges, where wind uplift is strongest.

Galvanized or Painted Purlins?

Pre-galvanized purlins: made from zinc-coated steel coil. Better corrosion resistance, no on-site painting, and the standard choice for modern PEB buildings.
Painted (red oxide or enamel) purlins: lower initial cost, but need repainting and are more likely to rust in humid or coastal areas.

Don't Forget Sag Rods and Bracing

Purlins are slender, so they need help to stay straight and stable:

Sag rods stop purlins from twisting and sagging along the roof slope.
Flange braces connect purlins to rafters and stop the rafter from buckling.
Purlin cleats must be properly bolted using structural nut bolts of the right grade.

Common Purlin Mistakes

Choosing purlin size by price instead of by design calculation.
Skipping sag rods to save cost.
Using too-short laps on Z purlins, which removes the benefit of continuity.
Spacing purlins too far apart for the roof sheet thickness.
Welding galvanized purlins on site without touching up the coating.

Buy the Right Purlin for Your Building

City Steel Corporation designs and supplies C and Z purlins cut to length for each project, together with steel rafters, columns and roof sheets. Our engineers check the purlin size, spacing and bracing for your exact span and location, so your roof is safe without overspending on steel.

Need purlins or a complete steel building? Call 01711-787556 or request today's purlin price.

Frequently Asked Questions

Which is better, C purlin or Z purlin?

Z purlins are usually better for long, multi-bay roofs because they can be lapped for continuity. C purlins are ideal for wall girts, canopies and short single spans.

What purlin size do I need for my factory roof?

It depends on bay spacing, roof load and wind zone. Many factory roofs use 200 mm to 250 mm deep Z purlins, but the final size must be confirmed by a structural engineer.

How is purlin price calculated?

Purlin price is mainly based on weight (per kg or per running metre), which depends on depth, thickness and coating. Ask for a quotation on the designed section, not just the cheapest size.

C and Z Purlin Guide: Sizes, Spacing and How to Choose in Bangladesh